High torque reduced starting current electric motor

Abstract

An AC squirrel cage motor runs with a rotor current frequency equal to the slip of the motor times its excitation frequency. It is desirable to reduce the rotor current on start up by means of an increased resistance, and then to run the motor with a reduced resistance once it is started. This invention takes advantage of the skin effect of high frequency currents by increasing rotor end ring resistance during startup. After the motor has started up, and has reached full speed, the rotor frequency is reduced to merely the slip multiplied by the stator frequency. The skin effect on the rotor current is less, and the motor runs with reduced resistance.

I claim:  
     
       1. A squirrel-cage motor, comprising, 
       a stator;  
       a rotor disposed within said stator defining an axis of rotation and two axially facing ends of said rotor;  
       said rotor also comprising a plurality of axially extending rotor bars circumferentially spaced about said axis;  
       said rotor further comprising an end ring on each end of said rotor, each end ring having an inner portion and an outer portion, wherein the outer portion has higher electrical resistance than the inner portion, and wherein the end rings are electrically connected to the rotor bars.  
     
     
       2. A motor according to claim  1  wherein the outer portion of the end rings is a material of higher electrical resistance than the inner portion. 
     
     
       3. A motor according to claim  1  wherein the rotor bars form two cages, an inner cage and an outer cage, and the inner cage is electrically connected to the inner portion of the end rings, and the outer portion is connected to the outer portion of the end rings. 
     
     
       4. A motor according to claim  3 , wherein the rotor bars of the outer cage are made of material with higher electrical resistance than the inner cage. 
     
     
       5. A motor according to claim  3 , wherein the inner cage and the outer cage are electrically connected.